High-strength steel (HSS) and ultra-high-performance concrete (UHPC) offer significant potential in structural engineering due to their superior strength. However, existing design codes often limit their use in steel-concrete composite columns due to the brittleness of UHPC and strain incompatibility issues between HSS and concrete. This study aims to improve the performance of both HSS and UHPC in concrete-encased steel (CES) columns by enhancing the strain capacity of UHPC and reducing its brittleness through the incorporation of steel fibers, while optimizing confinement with carbon fiber reinforced polymer (CFRP) and stirrups. The compressive behavior of CFRP-confined UHPC-encased Q690 HSS (CFRP UHPC-Q690 CES) stub columns was investigated, considering various parameters, including CFRP layers, stirrup spacing, steel fiber content, and steel section configuration (H and L sections). Results indicate that CFRP confinement enhanced the columns’ strength by approximately 7–13 % and improved ductility by up to about 19 %, while its effect on ductility was negligible for specimens with L-shaped steel sections. The incorporation of steel fibers further increased the ultimate resistance by approximately 8–11 %. In addition, high stirrup volumetric ratios, combined with CFRP confinement, enabled UHPC-Q690 CES columns without steel fibers to achieve plastic sectional resistance, thereby enhancing axial resistance predictions in accordance with the EC4 and JGJ 138 design codes. An analytical model based on strain compatibility was developed to predict the axial behavior of CFRP UHPC-Q690 CES columns, showing close agreement with the experimental results. |
